This invention relates generally to fluorescent lamp power supplies and particularly to battery driven power supplies for fluorescent lamps.
Fluorescent lamps are well-known for their efficiency in converting electrical energy into light energy. Recently, small fluorescent lamps have been used in portable equipment that is battery operated. A fluorescent lamp power supply generally includes a form of DC to AC converter and a step-up transformer for developing a relatively high voltage alternating current from the battery, since the fluorescent lamp requires a significant voltage for operation. Such power supplies are well-known in the art.
The fluorescent lamp has also found acceptance as a "back light" for liquid crystal displays (LCDs), especially LCDs used in connection with small "lap-type" computers. In such use it is desirable to be able to control the brightness of the fluorescent lamp to adapt the LCD display for use under a wide range of environmental lighting conditions. Brightness controls for fluorescent lights have not been available except for large industrial fluorescent lights. In that environment, the brightness controls involve very complex circuit arrangements. Another problem exists with portable fluorescent lamps, namely, that the light output from the fluorescent lamp will diminish with a reduction in the battery supply output voltage. Thus, as the batteries inevitably become weaker with use, the light output of the fluorescent lamp diminishes. Accordingly, there is a need in the art for a fluorescent lamp power supply that solves the above-noted deficiencies.
According to one aspect of the invention, a fluorescent lamp power supply is provided that is capable of controlling the brightness of a fluorescent lamp over a wide range. According to another aspect of the invention, a fluorescent lamp power supply is provided that compensates for changes in the power supply or line voltage to maintain the light output of the fluorescent lamp substantially constant. In apparatus for carrying out both aspects of the invention, a novel pulse width modulator (PWM) circuit is utilized that is readily fabricated from a dual comparator integrated circuit chip.